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8/2/2005 4: S OUND WAVES IN TUBES I NTRODUCTION So far we have studied oscillations and waves on springs and strings. We have done this because it is comparatively easy to observe wave behavior directly in these media. Sound is also a wave. In this lab we will study the behavior of sound waves in tubes. It is important that you compare the things you observe in this lab to your observations of the more easily visible waves in previous labs. Wave behavior is universal. A. T RAVELING W AVES IN T UBES 1. Getting Ready Measure the length of your tube to the nearest millimeter. Then glue the glass plate to the one end of the tube with rubber cement. L = 2. Hooking up the Pulse Generator You can make pulses of sound with a pulse generator . You are provided with a pulse generator which is similar to the function generator you have been using, except that it only generates square pulses of various widths and spacings. a. Hook the VARiable output of the 4001 Pulse Generator to the speaker. b. There are two time adjustments on the pulse generator: Pulse Spacing and Pulse Width. Pulse Width Pulse Spacing Each one has a control knob with a course setting (which clicks into place) and a fine setting (which is continuously variable). Set the Pulse Spacing to 100 ms and turn the fine adjustment fully clockwise. Set the Pulse Width to 10 µ s and turn the fine adjustment to 10 o’clock. Turn on the pulse generator and turn the Amplitude knob to 10 o’clock. You should hear a soft clicking sound coming from the speaker. 3. Looking at the Pulse on the Scope a. Connect a BNC/BNC cable from the TTL output of the generator to the TRIGger input on the scope and set the Trigger Select to EXTernal. Set the sweep rate to
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P108 Lab 4: page 2 0.1 ms/div and the Trace MODE to NORMal. b. Connect Banana/Banana leads from the speaker to the CH1 input and ground. Set CH1 to 1 Volt/div and the input selector switch to AC. Look at channel 1. You should now be able to see the voltage pulse created by the pulse generator if you turn the intensity all the way up. Adjust the fine control of the Pulse Width until the pulse is 0.1 ms long and adjust the Amplitude until the height of the pulse is 1.0 V. The pulse will look like this: 1 V 0.1 ms c. Connect the BNC terminal from the microphone to CH2. Set CH2 to 20 or 50 mV/div and the input selector to AC. Switch the scope to look at CH2. Turn on the mike and then hold the mike near the speaker. You should be able to see the pulse as detected by the microphone. This sort of very abrupt, square pulse is quite difficult for a speaker to reproduce, so the pulse you see detected by the microphone will have a more complicated shape, going sharply up but then overshooting and going slightly negative. Consider only the very first part of the pulse. A typical shape for the detected pulse is shown below. If the first part of the pulse goes negative, switch the leads going into the speaker to make it positive. Positive
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